[x86] Fix a regression introduced with r213897 for 32-bit targets where
we stopped efficiently lowering sextload using the SSE41 instructions
for that operation.

This is a consequence of a bad predicate I used thinking of the memory
access needs. The code actually handles the cases where the predicate
doesn't apply, and handles them much better. =] Simple fix and a test
case added. Fixes PR20767.